Recent advances in the technology using hydrogen as the energy source have become striking, for example, as in the widespread use of fuel cell. In this field, various improvements in the materials themselves of storage containers, pipes and the like to handle hydrogen have been contemplated, as shown in the technology to store hydrogen under high pressure. The adverse effect of hydrogen on the metal materials has been long studied in the research of corrosion. For example, hydrogen gas generated by cathodic reaction in etching solutions induces cracking as a result of stress concentration. Further, hydrogen is held by adsorption on the surface of the intermediate and precipitate, or hydrogen diffuses into the material through the portions around flaws and is accumulated there to make the portions brittle, to advance the cracking in the material and then result in fracture. The problem of hydrogen embrittlement of metal materials such as steel and the like, i.e., the process where the metal materials lose their ductility following the diffusion of hydrogen into the metal materials has been especially discussed recent years. The progress of hydrogen embrittlement will bring about serious results, i.e., cracking of the metal material and the like. The cracking of metal material by the hydrogen embrittlement is called delayed fracture phenomenon. The delayed fracture is also known as static fatigue where a high-strength member placed under the application of static tensile stress suddenly causes brittle fracture after a length of time. The cause of the delayed fracture in the high-strength member is considered to be hydrogen introduced into the material in the course of manufacturing process thereof or during the operational use thereof. Hydrogen is more likely to diffuse into a metal material of which the lattice vacancy density becomes higher by the plastic deformation, so that hydrogen is accumulated around the portions where the tensile stress is concentrated, that is, the screwed portions, corrosion pits and the like, to induce the fracture, that is, the so-called hydrogen embrittlement of the metal material. Generally, hydrogen occluded in metal, notably in steel, has little effect on the yield strength and tensile strength of the metal, but tends to cause the ductility and toughness of the metal to deteriorate. Accordingly, great attention should be given to hydrogen, especially in high-strength steel. This is because the material for the metal member becomes more susceptible to hydrogen embrittlement as the strength of the metal member is made higher.
There are few researches or studies about hydrogen embrittlement discussed from the viewpoint of tribology. The technology of fuel cell or the like where hydrogen is used as the energy, however, always involves transfer of hydrogen, and mechanical members, for example, relating to travelling are also accompanied by transfer of hydrogen as a matter of course. The compressor, which is one of the typical examples has rolling element bearings, sliding bearings and the like as the tribological elements. Some measures are essential to protect those mechanical members and the metal materials therefor from hydrogen embrittlement, but sufficient countermeasures are not taken at the present stage.
In the field of automobile electrical equipment or auxiliaries thereof, hydrogen embrittlement in the rolling element bearing has become a problem, and this problem has been handled by improving the properties of grease to be employed. For example, addition of an oxidizer for passivation to the grease is proposed in order to inhibit a catalytic action of the metal surface newly appearing as a result of the wear (e.g., JP 3-210394 A). According to the above-mentioned proposal, the metal surface is oxidized to inhibit the catalytic action thereof, thereby preventing the generation of hydrogen that would be caused by decomposition of the lubricant. Also, use of a phenyl ether synthetic oil as the base oil for grease is proposed to prevent the generation of hydrogen caused by decomposition of the lubricant (e.g., JP 3-250094 A). There is another proposal that a particular thickener, an oxidizer for passivation and an organic sulfonate are added to a particular base oil (e.g., JP 5-263091 A). Further, it is proposed that azo compounds capable of absorbing hydrogen be added to the grease used for metal materials required to have tribological properties and for a variety of members, in particular, to the grease to be filled into the bearing used at a portion subject to entry of water (e.g., JP 2002-130301 A). In addition, for the purpose of obtaining a long-life rolling element bearing which does not produce the problem of hydrogen embrittlement-induced flaking even when water permeates through the bearing, a grease composition is proposed where a fluorinated polymer oil, polytetrafluoroethylene as the thickener and an electroconductive material are added to a base oil (e.g., JP 2002-250351 A).
There are some mechanical members operated by a sliding motion, not by the rolling motion, and the life of those members is limited by wear and seizure instead of flaking. The representative examples of those mechanical members include journal bearing (sliding bearing), piston, screw, rope, chain and the like.
The fatigue life used herein is a useful life of metal determined by rolling fatigue. To use the mechanical members over a period of the above-mentioned fatigue life, thickening of the lubricating oil film is a conventional means.
However, the cause of the flaking occurring in an atmosphere of hydrogen is considered to be hydrogen diffusing into steel to lower the mechanical strength of the steel material (Endo, Dong, Imai, Yamamoto “Study on Rolling Contact Fatigue in Hydrogen Atmosphere” Journal of Japanese Society of Tribologists Vol. 49, No. 10). In light of this, the flaking cannot be prevented only by thickening the lubricating oil film.
JP 2007-262300 A describes that the addition of rust inhibitor such as organic sulfonates, carboxylates, thiocarbamates and the like can effectively prevent the flaking from occurring in an atmosphere of hydrogen. It is believed that a coating of the rust inhibitor can block the entrance of hydrogen.